Sensors that detect and monitor human surroundings and communicate acquired physical data, such as pressure, shear, strain, and other physical inputs, enable the creation of a number of useful devices having application from consumer electronics to healthcare and biomedical monitoring. The increasing demand to measure and store a wide range of sensor data places significant demand on the design and development of new sensor technologies. Moreover, the need to incorporate these sensors into wearable articles that are in intimate contact with the body requires that the sensors and their related assemblies be lightweight, flexible, an offer extremely favorable performance parameters including long-term stability, high sensitivity, low profile dimensions, low signal-to-noise ratio, and extremely rapid response rates. Where the sensors are incorporated into a wearable item, the structures must be suitable for integration with fibrous or cloth substrates, such as fabrics ordinarily used for garments. Fabrics offer an attractive platform for wearable sensors because fabrics have high degrees of deformability, conformability, long-term stability, can place high performance sensors in intimate contact with the body potentially without compromising either the wearability of the fabric substrate or the performance of the sensor.
The design of the such sensors must take into account the structural characteristics of the fabric material in which the sensor will be located. A sensor that it is in flexible or cannot withstand repetitive motion would not be usable in combination with fabric used in article of clothing that is expected to be repeatedly exposed to variable pressures and sheer forces generated during ordinary wear. Similarly, a sensor that is inoperative, or which loses sensing capacity, in the presence of moisture would not be an effective sensor for many article of clothing that are expected to get wet in ordinary usage. Accordingly, an ideal sensor must balance competing needs: 1) the sensor must satisfy a number of technical sensing parameters to generate high quality data, such as small changes in pressure, to make the sensor useful for a wide variety of purposes, and 2) the sensor assembly must be able to be integrated with the fabric material while remaining the capability to collect high-quality sensor data under all of the external conditions imposed on the fabric material, including for example repetitive motion, stretching, bending, and twisting forces as might be expected for ordinary fabric.
Moreover, flexible and wearable sensors must provide accurate and reliable sensing data without compromising the natural movement and comfort of the user, otherwise the data would not be useful and utility of the sensor in ordinary wearable applications would be greatly diminished. An additional challenge is the ability of the sensor to be incorporated in a flexible, wearable substrate during manufacture of the combination of the fabric and the sensor such that neither the sensing function nor the fabric quality is compromised during manufacture and that the resulting article remains capable of high quality data collection and transmission.
Fabric offers an ideal platform for wearable sensors to detect physiological and health data. Because of the intimate contact with the skin, sensors can be incorporated directly into wearable clothing fabric with little impact on the normal activities of the user. A fabric-based sensor works best when it is held close to the skin. To achieve close conformity to the skin, a wide variety of polymer materials have been used due to ease of manufacture and inherent flexibility. As long as the technical capabilities of the sensing element are compatible with the task of data collection in a fabric platform and subsequent transmission to a data processing unit, a variety of clothing items can be modified to contain sensors to detect data. Depending on the construction of the fabric article, such as a piece of clothing, and the particular the physiological data that is to be collected, the integration of pressure sensors into the fabric may have little or no impact on wearability.
Existing pressure sensors tend to be based on three classic sensing mechanisms: 1) a flexible conductor mode, 2) a piezoelectric-based detection mode, and 3) a parallel-plate capacitance configuration. Currently, the flexible resistor approach has been the primary modality for wearable applications but is plagued by slow mechanical response times and low signal-to-noise ratios that limit the signal bandwidth and resolution. Furthermore, many materials are only available for dynamic force measurement, measurable changes in pressure over time, and are not suitable for static readout, measuring pressure without necessary movement that causes pressure differential changes. The sensor wearable flexible platform combination must also account for electrical properties of the body itself that tend to limits the ability of parallel-plate capacitance based sensors to make fine pressure measurements.
Accordingly, flexible and wearable physical sensing platforms must satisfy a number of challenging, and often conflicting, design and performance parameters to permit real-time, static and dynamic, high signal/low noise, and overall high-performance data detection and processing to facilitate the practical application of new high-tech sensors to electronic, healthcare, and biomedical applications. The development of flexible and wearable sensors demands innovation in the material science of the sensor element, the fabrication process of sensor, and the creation of a sensor assembly to take advantage of the high-performance capabilities of the sensor element, while maintaining the ability to integrate the sensor assembly into a fabric-based platform.